Something's wrong here. I haven't kept up, but I assume you're talking about a SBC. The main cap bolts are 7/16" diameter, I can't remember whether they're 14 or 20 TPI, but they're 7/16" diameter (0.4375"). The drill to use for threaded inserts for a finished 7/16" thread is 37/64" (0.578125"), so you will not be able to use the main cap as a guide.

If I were doing this in a home garage and didn't want to haul the block to the machine shop, I might proceed thusly:

1. Use a bottoming tap to clean up the threads in the hole and make sure I had maximum thread length in the hole.
2. Get myself a used rear main cap and mount it on the drill press to drill out the bolt hole with the 37/64" drill bit.
3. Mount the used cap on the block, using the bolt on the other side of the cap to secure it.
4. Drill the hole.
5. Remove the main cap, mount it on the drill press again and drill the hole out to 0.625", the diameter of the tap you will use.
6. Secure the cap to the block again and use it for a guide to keep the tap lined up. If the tap is too short to start in the hole through the guide hole, grab your Sawzall and slice off a slab of material from the cap, making it thinner.
7. Install the insert with loctite after cleaning all lube from the hole.
8. Get yourself a set of main cap studs from Milodon or other premium supplier and stud all the caps.